Growth hormone (GH) is a protein hormone produced by the anterior pituitary. GH has multiple somatogenic and metabolic functions including promotion of skeletal growth and promotion of the differentiation of fat cells and chondrocytes. GH exerts its actions on many target tissues via binding to the growth hormone receptor (GHR). GH exerts many of its effects indirectly by stimulating the liver to produce insulin-like growth factor-I (IGF-I, also known as somatomedin C). GH may also stimulate IGF-I production in tissues other than the liver [Rechler et al. (1987) N. Engl. J. Med. 316:941]. IGF-I binds to IGF-I receptors on target cells in a variety of tissues and thereby regulates growth and differentiation (e.g., increased chondrogenesis leading to skeletal growth, increased protein synthesis and cell proliferation leading to an increase in extraskeletal growth). GH also exerts a direct effect by binding to the GHR in some tissues (e.g., stimulation of erythropoiesis) [Hughes and Friesen (1985) Ann. Rev. Physiol. 47:469 and Isaksson et al. (1985) Ann. Rev. Physiol. 47:483].
The GHR is a member of the cytokine/GH/prolactin receptor superfamily. Receptors in this superfamily transduce signals by association and activation of Janus tyrosine kinases (JAK kinases). In particular, the binding of GH to the GHR induces tyrosine phosphorylation and activation of JAK2 tyrosine kinase and tyrosine phosphorylation of the GHR [Xu et al. (1996) J. Biol. Chem. 271:19768 and Wang et al. (1996) Mol. Endocrinol. 10:1249]. GH binding to the GHR also induces tyrosine phosphorylation of cellular proteins termed signal transducers and activators of transcription (STATs), including STAT1, STAT3 and STAT5. GH has been shown to induce the association of STAT5 with the GHR in a GH-dependent manner; STAT5 is phosphorylated in response to GH binding and it is believed that JAK2 is the kinase that carries out this phosphorylation event. Two STAT5 homologs, STA5A and STAT5B have been identified in the mouse and STAT5A, but not STAT5B, undergoes GH-dependent tyrosine phosphorylation [Xu et al. (1996), supra]. These findings demonstrate that the GH-dependent activation of JAK-STAT pathways plays an important role in the GH signaling system that leads to GH-induced biological responses.
Circulating GH is complexed with one of two binding proteins, a high-affinity binding protein or a low affinity binding protein (.alpha..sub.2 -macroglobulin) [Baumann and Shaw (1990) J. Clin. Endocrinol. Metab. 70:680]. The high-affinity binding protein is termed growth hormone binding protein (GHBP). GHBP corresponds to the extracellular hormone-binding domain (i.e., the ectodomain) of the membrane associated GHR [Peeters and Friesen (1977) Endocrinol. 101:1 164; Baumann et al. (1986) J. Clin. Endocrinol. Metab. 62:134; Herington et al. (1986) J. Clin. Invest. 7:1817; Leung et al. (1987) Nature 330:537; Herington et al. (1986) Biochem. Biophys. Res. Commun. 139:150; Hocquette et al. (1990) Endocrinol. 127:1665 and Smith and Talamantes (1988) Endocrinol. 123:1489]. In humans and rabbits, GHBP is generated by proteolytic cleavage of the GHR to release the extracellular domain [Leung et al., supra; Trivedi et al. (1988) Endocrinol. 123:2201; and Sotiropoulos et al. (1993) Endocrinol. 132:1863]. In rodents, GHBP is generated by alternative splicing of an RNA transcript which also gives rise to mRNA encoding GHR [Smith et al. (1989) Mol. Endocrinol. 3:984 and Baumbach et al. (1989) Genes Devel. 3:1199]. GHBP is highly conserved through evolution. In addition, similar circulating ectodomains (i.e., extracellular domains of membrane-associated receptors) exist for several other receptors in the cytokine receptor family.
GHBP prolongs the half life of plasma growth hormone (GH) by effectively competing with GHR for ligand. By controlling free GH levels between secretory "peaks", GHBP appears to modulate the bioavailability of free GH. Given that GHBP and GHR are often co-expressed, serum GHBP levels have been extrapolated to estimate tissue concentrations of GHR [Baumann (1993) Proc. Soc. Exp. Biol. Med. 202:392 and (1994) J. Endocrinol. 141:1].
Plasma GHBP levels are altered in a variety of pathological states. In several conditions characterized by GH-resistance, GHBP levels are decreased (e.g., fetal life, senescence, malnutrition, insulin-dependent diabetes, hypothyroidism, liver cirrhosis, chronic renal failure, Laron syndrome, Pygmy dwarfism), whereas in overnutrition (obesity), GHBP levels are elevated [Baumann (1993), supra; Baumann (1994), supra; Baumann and Mercado (1993) Nutrition 9:547; and Maheshwari et al. (1996) J. Clin. Endocrinol. Metab. 81:995]. Extremely low or extremely elevated levels of GHBP can occur on a familial or genetic basis [Baumann et al. (1987) J. Clin. Endocrinol. Metab. 65:814; Daughaday and Trivedi (1987) Proc. Natl. Acad. Sci. USA 84:4636 and Rieu et al. (1993) J. Clin. Endocrinol. Metab. 76:857].
Laron syndrome, or GH insensitivity syndrome, is an example of mutations in the GHR gene resulting in absence or dysfunction of the GHR [Godowski et al. (1989) Proc. Natl. Acad. Sci. USA 86:8083; Berg et al. (1993) Am. J. Hum. Genetics 52:998; and Amelselem et al. (1993) Human Mol. Genetics 2:355]. When, as is the case in most families, the mutation is located in the extracellular domain of the GHR, affected patients also have absent, very low, or dysfunctional GHBP [Baumann et al. (1987), supra; Daughaday and Trivedi, supra; Rosenbloom et al. (1990) N. Engl. J. Med. 323:1367; and Savage et al. (1993) J. Clin. Endocrinol. Metab. 77:1465]. About 25 different mutations (partial gene deletions, nonsense and missense point mutations) have been described in various families [Berg et al. (1993), supra; Amelselem et al. (1993), supra; Woods et al. (1996) J. Clin. Endocrinol. Metab. 81:1686; Duquesnoy et al. (1994) EMBO J. 13:1386; Kou et al (1993) J. Clin. Endocrinol. Metab. 76:54; Ayling et al. (1996) Progr. 10th Internatl. Congr. Endocrinol., 748; and Goddard et al (1995) N. Engl. J. Med. 333:1073].
Patients with Laron syndrome are characterized by a severe postnatal growth failure and markedly reduced adult height. In addition, clinical findings in patients with Laron syndrome include obesity, normal to high levels of circulating GH, low levels of IGF-1, low or no GHBP, resistance to exogenous GH treatment and hypoglycemia [Savage et al. 1993, supra; Rosenfeld et al. (1994) Endo Rev. 15:369]. Currently, the only form of treatment for Laron syndrome is infusion of recombinant human IGF-1 [Walker et al. (1991) N. Engl. J. Med. 324:1483]. No mammalian models for this disorder are available for the development of alternative therapeutic modalities.
The art needs a mammalian animal model of Laron syndrome to provide a model system for the screening of therapeutic compounds and regimens to provide improved therapy for patients suffering from GH insensitivity syndrome (i.e., Laron's syndrome).